1. Field of the Invention
This invention relates generally to optical receivers and more particularly to optical transimpedance receivers with compensation networks in a non-linear feedback loop.
2. Description of the Related Art
An optical receiver typically includes a photodetector and a low noise preamplifier as a front end, and an equalizer, post amplifier and filter as a linear channel. This arrangement converts a modulated optical signal into an electrical signal. Further processing of the signal can then be done to recover whatever information had been impressed on an optical carder. The front end of the receiver often consists of a photodiode as the photodetector and a transimpedance amplifier as the preamplifier. This forms a transimpedance front end where a load resistor is replaced by a large feedback resistor, and negative feedback around a wideband amplifier is used to obtain increased bandwidth.
The capacitive input impedance of the low noise preamplifier causes a phase shift of 90 degrees at high frequency, and the inverting amplifier contributes a phase shift of 180 degrees. Optimally, a phase margin of about 45 degrees needs to be maintained. This allows for a maximum additional open-loop phase shift of only 45 degrees up to the frequency at which the open-loop gain becomes less than unity. This small, tolerable phase shift limits the gain that can be included within the feedback loop and it is usually not possible to obtain the desired preamplifier bandwidth if a large feedback resistor is used. Limiting the size of this feedback resistor will lower the sensitivity but, in exchange, will have greater simplicity and much wider dynamic range. In order to maintain the sensitivity of the signal, transimpedance front ends normally give up some of the dynamic range. One prior art method of overcoming this loss of range was by forward-biasing the gate-source diode of the front-end transimpedance amplifier to shunt the feedback resistor. This is described in "PIN-GaAs FET Optical Receiver With a Wide Dynamic Range" by B. Owen, Electronic Letters Vol. 18 No. 4, July 1982, pages 626, 627. Although this reduces the loss of the dynamic range, peaking will occur when the diode is switched on, causing instability in this non-linear circuit.